In North America, hazardous environments fall into different categories. Class 1 Division 1 covers hazardous environments in which a hazard, such as an explosive gas, is continuously or frequently present. Class 1 Division 2 covers hazardous environments in which the hazard potentially can be, but is only rarely, present. An arbitrary threshold level has been established of less than 10 hours a year, which serves as the dividing line between Division 1 and Division 2.
In Division 1 environments, circuit breaker panels are used which have explosion proof housings made from cast aluminum with specific tolerances engineered to contain any explosion. A flange is provided around an access opening to the housing. A closure door is secured by dozens of bolts to the flange, in order to prevent the closure door from being blown off the housing by the explosion. The width of the flange is selected so that any hot gases escaping past the flange have cooled sufficiently to avoid igniting gases in the outside environment by the time they exit the housing. The weight of these explosion proof housings vary with their size, but in all cases they weigh hundreds of pounds and a support structure must be provided that can support such weight. In view of this approach North American Division 1 circuit breaker panels are expensive and cumbersome to access.
Europe has adopted similar categories to Division 1 and Division 2, which are termed “Zone 1” and “Zone 2”. Their Zone 1 standards are more stringent. Each electrical component must have its own self contained explosion proof casing. These explosion proof electrical components are placed into carbon impregnated enclosures which can be grounded. They use multiple housings which are assembled in a modular fashion. The European Zone 1 circuit breaker panels are even more expensive than those used in North America. European Zone 2 installations use the same circuit breaker panels as are used for Zone 1.
In an effort to address the above described problems with expense and access, there has been a movement to use two panels in tandem in North American Division 2 environments. One panel has a housing which is not explosion proof and the other panel has an explosion proof housing. Only those circuit breakers and other electrical components which may cause arcing or sparking during the course of their operation, are placed into the explosion proof housing. Providing two housings reduces access problems, as only circuit breakers and other arcing and sparking electrical components must be isolated within the explosion proof housing. It does not, of course, reduce the size and space requirements as two housings are used.
Although using one explosion proof panel and one panel that is not explosion proof has reduced expense and access problems, the industry is still looking for a better approach to Division 2 circuit breaker panels.